The invention is in the field of sputtering apparatus and methods and pertains more specifically to sequential deposition of disparate material layers from a composite target assembly.
Sputtering is a commercially preferred process for deposition of homogeneous films of controled thickness. In particular, contemporary media for information storage purposes is generally fabricated through sputter deposition of materials for magnetic, or optical storage.
It is especially desirable to form a multi-layered medium such as shown in FIG. 1 where alternating films 6 and 8 of, for example, cobalt and platinum respectively, are deposited on a substrate 4. These layers may approach aggregate thickness of the order of 102 to 103 angstroms in many applications, with minimum single layer thicknesses of 2 to 3 angstroms. Multilayered structures of this type exhibit excellent magnetic properties and particularly find application in very high density information storage wherein the magnetic vector of a stored bit is substantially directed toward the normal to the plane of the layered medium. An initial underlayer(s) and final layer(s), not shown, complete the layered structure for its intended purpose. The sequence of adjacent layers (xe2x80x9cbilayersxe2x80x9d) may number in the neighborhood of 15-30 in typical storage applications. It is desirable, for specific applications, that the thickness of layers 6 and 8 be independently controllable and put down without cross contamination during deposition. Layered sequences involving more than two sputterable materials may be required in selected applications.
Apparatus is known that employs multiple independent discrete sputter sources within a common vacuum housing for deposition of a series of distinct layers via reactive processes. A representative apparatus of this type disposes four DC magnetrons azimuthally about the axis of a rotatable pallette facing the array of sputter sources for supporting a number of substrates. Alternating in position between the DC magnetrons are (four) reactive gas plasma cells for controlled oxidation of a sputtered aluminum layer to produce alumina layers on the substrates. The pallette rotation accomodated either a continuous or indexed rotational mode. Systems of this general description were available under the designation, ALX 1000 from the assignee of the present invention.
Another prior art sputtering system includes a multi-species sputtering source including concentric target rings coaxially disposed in relation to a concentric coil magnetron. In this system, selection of sputtering target is accomplished by magnetic manipulation of a plasma to the ring shaped region proximate to one of either the inner or outer target ring. See U.S. Pat. No. 5,705,044.
It is also known in prior art to employ a sputtering target comprising different area portions of distinct sputtering materials for the deposition of a thin film alloy of desired composition onto a workpiece. See xe2x80x9cHandbook of Sputter Deposition Technologyxe2x80x9d, K. Wasa and S. Hayakawa (Noyes Publications, 1992).
Although an information storage medium is the exemplary application for the present invention, other applications include components for optical circuits and specialized optical components, and for fabrication of thin film laminated structures of both periodic and aperiodic form.
The present invention allows fabrication of these multi-layered structures with excellent control of sputter deposition featuring an exceptionally high freedom from cross contamination of individual deposited layers, without excesive handling of the substrate, obtaining commercial efficiency characterized by rapid throughput.
In the present invention, a large area sputtering target is formed of distinct surface area portions. Each adjacent pair of surface area portions comprise distinct sputterable materials, as for example a cobalt portion adjacent to a platinum portion. A workpiece is disposed proximate the target and capable of relative displacement opposite different sputterable areas of a sputter target comprising distinct surface portions. Linear dimensions of the workpiece do not exceed the dimensions of a given sputterable portion of the target so that the workpiece can be positioned in its entirety opposite such selected surface portion or relatively rotated (or, in an alternate geometry, translated) continuously with respect to the target. A sputtered film from the proximate sputterable target portion is deposited on the workpiece in conventional fashion. In a preferred embodiment, the projection of the workpiece on the target is stationary as the target moves relative to the facing (planar) workpiece. In a dynamic mode of operation, sputter deposition is enabled during selected intervals of relative motion when the workpiece and a selected portion of the target are juxtaposed. In a static mode of operation the selected target is indexed to a position opposite the workpiece and sputtering is enabled for the desired deposition time, after which the target is repositioned (indexed) for the next deposition. Sputtering is enabled, for example by gating on a high voltage power supply to excite a spatially well defined plasma discharge only during a desired interval while the selected target portion is proximate the facing well defined plasma projection on the target. The workpiece, spaced from the other side of the plasma discharge, receives the sputtered flux from the target. In like manner, the sputtering discharge is gated off as the moving well defined projection of the plasma discharge on the target (dynamic mode) approaches a boundary delimiting an adjacent target portion. The discharge may be terminated earlier, if the sputtering interval is sufficient to yield the desired thickness film on the workpiece in a single pass across the selected target portion. This operation is repeated in respect of the next adjacent target portion to produce alternating films of the respective materials on the workpiece.
The target is disposed within a housing into which the workpiece is introduced. In a preferred geometry, the workpiece is stationary and faces a rotating target wheel comprising at least two sectors, for example, approximately 180xc2x0 sectors of cobalt and platinum, respectively. A magnetic field is axially aligned with the mutual perpendicular between workpiece and target to sustain a plasma discharge upon application of a suitable potential between the target and housing. The magnetic field is carefully designed to provide a field distribution roughly occupying a generally cylindrical region sharply delimited in its radial extent. The plasma discharge is distributed in a region between the target and the workpiece and exhibits a well defined projection on the target by virtue of the properties of the magnetic field distribution. An apertured shield is disposed between workpiece and the target, surrounding the projection of the plasma on the target and protecting the target from cross contamination by back-sputtering and generally limiting sputter deposition on surfaces other than the workpiece. When the projection of this aperture on the target wheel is entirely within the desired sector, an enabling logic level causes the power supply to excite a plasma discharge and in like manner, the de-excitation gate signal is generated as the leading edge of the plasma projection on the target approaches the boundary separating adjacent sectors. The angular position of the target wheel is derived from a shaft position encoder providing an input to a sputter module controller. The scale of achievable thickness deposited for a single pass depends upon the sputtering material, rotation rate, pressure, power dissipated in the plasma, possible biases applied to the workpiece, and duty cycle(s) of such plasma excitation. A plurality of full rotations may be employed to intersperse desired ratios of thickness for adjacent layers.
In another embodiment, the workpiece is translated in linear motion with respect to an array of target portions with appropriate position encoding to inform a controller in like manner to the above, for deposit of the desired sputtered films.